This invention relates to a control circuit and an associated power circuit for use with a mine shuttle car.
Mine shuttle cars are used in mining operations in general principally to transport coal or other mined materials. Due to the nature of this type of operation, the particular shuttle cars must function without producing any sparks which might cause fires or explosions. Consequently, the prior art shuttle cars generally operated from a cable which powers a DC traction motor.
The traction motors of the prior art shuttle cars were generally controlled by some form of a hand control knob. The control circuitry associated with them generally consisted simply of a series of resistors arranged in series with the traction motor. When the hand control was turned up to increase the speed, one or several of the resistors would drop out of the circuit thereby increasing the voltage across the motor and increasing its speed. Similarly, the reduction in speed was accomplished by turning the hand control so that one or more of the resistors were reinstated in the series circuit. These quantum jumps, however, do not produce a continuous flow of current to the traction motor. Consequently, the prior art control circuits necessarily placed an undue strain on the drive shaft and other mechanical devices associated with the shuttle car. As a further maintenance problem, the contacts in the hand control have a tendency to wear out and have to be replaced frequently. And finally, the amount of control the operator has over the speed of the motor is understandably somewhat limited by this type of control circuit. Nevertheless, despite these drawbacks, these shuttle cars with the prior art control circuits are widely used.